Jonathan L. Tilly, Ph.D.
Investigator, Vincent Center for Reproductive Biology
Professor, Department of Obstetrics, Gynecology and Reproductive Biology, Harvard Medical School
Affiliated Faculty, Harvard Stem Cell Institute
Brief Overview of Tilly Lab Research
The long-term goal of work in my laboratory is to improve women’s reproductive healthcare and overcome infertility by applying what we discover through basic and translational (preclinical) research. For over 16 years, one focus of my research group has been to understand the roles and regulation of physiological cell death or apoptosis in the mammalian ovary (Nature Rev Mol Cell Biol 2001 2: 838-48). Much of our work uses mouse models, which possess the advantage of relatively straightforward genetic manipulation. Since we started this work in the early 1990s, we have documented that premature ovarian failure and infertility resulting from conventional cancer treatments involves the activation of apoptosis in oocytes (Nature Med 1997 3: 1228-32; Genes Dev 1998 12: 1304-14; Mol Endocrinol 1999 13: 841-50). We then validated an anti-apoptotic small molecule that protects the ovaries from side-effect damage caused by conventional caner treatments as a means to preserve normal reproductive function (Nature Med 1997 3:1228-32; Nature Med 2000 6: 1109-14; Nature Med 2002 8: 901-2), and we have recently validated the efficacy of this small molecule and its mimetics to protect ovaries and natural fertility following radiotherapy through preclinical testing using rhesus monkeys as a model (Fertil Steril 2011 95: 1440-5).
In related work, we have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are toxic chemicals present at high abundance in the environment and in cigarette smoke, activate an orphan receptor-transcription factor (termed the aryl hydrocarbon receptor or AhR) in oocytes, leading to de novo expression of pro-apoptotic genes necessary for oocyte death (e.g., Bax) and early ovarian failure. Specifically, we reported that the AhR is abundantly expressed in oocytes, and that Ahr gene knockout in female mice leads to increased oocyte survival (Endocrinology 2000 141: 450-3). Moreover, in subsequent studies (Nature Genet 2001 28: 355-60; Endocrinology 2002 143: 615-20), we showed that the chemically-activated AHR protein induces Bax gene transcription in oocytes, and that expression of both the Ahr and Bax genes is functionally required for these chemicals to cause oocyte depletion and ovarian failure. Using a human ovarian xenograft model, we have further shown that an induction of Bax gene expression and apoptosis occurs in human primordial and primary oocytes following PAH exposure in vivo (Nature Genet 2001 28: 355-60). These data have been expanded on in a comprehensive study that details the global changes in expression of a large cassette of pro-apoptotic genes (in addition to Bax) in the ovaries following PAH exposure, and the requirement for p53 in working with the AHR to initiate oocyte apoptosis and follicle loss (Reprod Sci 2009 16: 347-56).
In addition to this work on pathological models of premature ovarian failure, we have shown that targeted inactivation of the pro-apoptotic Bax gene in mice prolongs ovarian lifespan into very advanced age, thereby eliminating the “mouse equivalent” of the menopause (Nature Genet 1999 21: 200-3). This animal model has allowed us to explore, for the first time, the impact of sustained ovarian function on the aging female body, and to decipher the contribution of age-related ovarian failure versus the aging process itself to the manifestation of various health complications often observed in women after the menopause (Proc Natl Acad Sci USA 2007 104: 5229-34). From these types of studies, we are building support for the concept that methods to achieve “ovarian replacement therapy” may offer novel ways to minimize the health risks faced by females as a consequence of ovarian failure due to either aging or insults.
It is important to emphasize that over the 8 years, our research efforts changed from those focused solely on ovarian cell death to now also include ovarian cell renewal, based on our studies that challenge one of the most basic doctrines in our field. Specifically, we have discovered the existence of female germline or oogonial stem cells that support oocyte and follicle production in the ovaries of adult female mammals. For example, we have shown that juvenile and adult female mice retain proliferative germline cells that, based on rates of oocyte degeneration and clearance, are needed to continuously replenish the oocyte-containing follicle pool (Nature 2004 428: 145-50). This line of study is now aimed at fully characterizing female germline stem cell function, identifying the existence of such cells in humans, and developing new stem cell-based strategies for enhancing female fertility and perhaps even delaying age-related ovarian failure.
Progress towards completion of these objectives is exemplified by a subsequent publication from our group documenting the presence of early germ cells in bone marrow and peripheral blood of adult female mice that are capable of generating immature oocytes in the ovaries of chemotherapy-sterilized or genetically-infertile adult female recipients following transplantation (Cell 2005 122: 303-15). Further, we have shown that bone marrow transplantation rescues long-term fertility in adult female mice exposed to sterilizing doses of chemotherapy (J Clin Oncol 2007 25: 3198-204). Likewise, infusions of young adult female bone marrow-derived cells into aging female mice postpones the age-related onset of infertility and markedly improves the postnatal survival rates of offspring conceived by infused aged females (Aging 2009 1: 49-57). These studies, coupled with our efforts to identify specific genes and pathways that regulate the activity of germline stem cells in adult mammalian females (Cell Cycle 2007 6: 2678-84), are providing the foundation for our ongoing development of a high throughput screening assay for identifying novel “oogenic” factors as potential therapeutics. Although this work was initially controversial as would be expected of a major paradigm shift (reviewed in Biol Reprod 2009 80: 2-12), independent confirmation and extension of these concepts is now available from many laboratories (e.g., Reproduction 2006 132: 95-109; Nature Cell Biol 2009 11: 631-6; Differentiation 2010 79: 159-70; see also Mol Human Reprod 2009 15: 393-8).
Finally, my laboratory has a considerable interest in defining the mechanisms that underlie, and accordingly how to prevent, the deterioration of egg quality with advancing maternal age. Numerous past studies have shown that chromosomal and meiotic spindle abnormalities become much more prevalent in oocytes with age, and are considered the major factors responsible for the increased incidence of infertility, fetal loss (miscarriage) and conceptions resulting in birth defects – most notably trisomy 21 or Down syndrome – in women over 35 years of age. This latter problem is compounded by modern fertility trends in that first birth rates for women 35–44 years of age in the United States have increased by more than 8-fold the past 4 decades. Using mice as a model, we reported several years ago that moderate reduction in dietary caloric intake (referred to as caloric restriction or CR) during adulthood remarkably extends female reproductive lifespan and improves the survival rates of offspring conceived and delivered by aged females (Aging Cell 2008 7: 622-9). In follow-up studies, we demonstrated that these benefits to female fertility were the result of vastly improved egg quality in aging females (Proc Natl Acad Sci USA 2011 108: 12319-24). Among other things, we found that adult female mice maintained under 40% CR did not exhibit aging-related increases in oocyte aneuploidy, chromosomal misalignment on the metaphase plate, meiotic spindle abnormalities, or mitochondrial dysfunction (aggregation, impaired ATP production), all of which occurred in oocytes of age-matched ad libitum-fed controls. We are now using this information to develop new strategies that target the bioenergetic potential in eggs as a means to maximize egg quality and improve outcome success in assisted reproduction.